The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a high coercivity ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic material (AFM). Additionally, a synthetic antiferromagnet (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used as the pinned layer. This results in an increase in the size of the pinning field so that a more stable pinned layer is obtained.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Early GMR devices were designed to measure the resistance of the free layer for current flowing parallel to the film's plane. More recently, devices that measure current flowing perpendicular to the plane (CPP) have replaced them. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. In contrast, in a CPP device, the resistance of the leads and of the other GMR stack layers dominate and must be as low as possible.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. Magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to ensuring that the free layer is a single domain and to ensure that the domain configuration remains unperturbed after processing and fabrication steps and under normal operation. For CIP devices this is usually accomplished by giving the structure a permanent longitudinal bias provided by two opposing permanent magnets located at the sides of the device.
As track widths grow very small (<0.2 microns), the above biasing configuration has been found to no longer be suitable since the strong magnetostatic coupling at the track edges also pins the free layer which drastically reduces the SV sensitivity. The solution to this problem that has been adopted by the prior art is illustrated in FIG. 1. Shown there is a magnetic read head whose bottom lead is also bottom magnetic shield 11 on which have been deposited pinning layer 13 and pinned layer 14. As noted earlier, the latter is typically a trilayer of two magnetically antiparallel ferromagnetic layers separated by a layer of an antiferromagnetic coupling material such as ruthenium.
Non-magnetic layer 16, which is usually copper, has been given the shape of a pedestal or disc over whose center the GMR stack has been formed. The latter consists of free layer 17 which has been given its stabilizing bias by bias ferromagnetic layer (BFL) 19. It is important that this stabilizing bias be provided by a magnetostatic field and not by exchange coupling so thin non-magnetic decoupling layer 18 has been inserted between layers 17 and 19. The magnetization of BFL 19 is stabilized by being contacted by a second pinning layer, antiferromagnetic layer 20 while layer 15 is insulating material to provide internal support for the structure.
Layer 21 is a high conductance layer that provides electrical connection to top magnetic shield 12 that also serves as the top lead. To simplify manufacturing the structure seen in FIG. 1, conductive layer 21 is given the same shape and area as the remainder of the GMR stack. Since the top lead/shield 12 is made of relatively high electrical resistivity material, there is a significant spreading resistance, associated with the passage of current from layer 21 into layer 12, which contributes to the overall series resistance of the CPP unit. An another significant contribution to the series resistance comes from layer 20 because of its relatively high resistivity and small cross-sectional area.
The following reference of interest was found during a routine search of the prior art: Frederick Hayes Dill et al, U.S. Pat. No. 6,023,395 2000.